The present invention generally relates to a raster scanning type display apparatus, and more particularly, to a display apparatus having an attribute controlling function.
Two typical techniques are known for using control codes called attributes to control the display modes of characters. The first is a technique using one character attribute for each character. Display devices which use this technique include those of the type which store character codes and character attributes alternately at successive locations of a memory and those of the type which store the character codes and the character attributes in separate memories or memory areas.
The second is a technique using field attributes. Each field attribute determines the display mode of a group of characters. In this technique, field attributes for groups of characters are stored in successive locations of a memory.
Generally, a display apparatus which uses either the first or second technique is constructed so as to enable only the use of either field attributes or character attributes but not the use of both of them. In consideration of such circumstances, there has been proposed a third technique which enables the use of both kinds of attributes. Namely, a technique disclosed in Japanese Published Unexamined Patent Application No. 55-78336 uses both field attributes and character attributes by using codes having the special format illustrated in FIG. 9. More specifically, the higher three bits B8-B10 of a code consisting of eleven bits B0-B10 are allocated as a character attribute, and the bit B7 is set to 0 or 1 to indicate whether the lower seven bits B0-B6 constitute a character code or a field attribute.
While the third technique is desirable to attain a versatile attribute control, the technique disclosed in the above Japanese Published Unexamined Patent Application No. 55-78336 has the problem that the memory is not effectively used. Namely, since each of the codes stored in successive storage locations includes a portion allocated as a character attribute, it is apparent that such character attribute portions in all the codes in the memory would be of no use and the spaces storing them would be wasted in a situation in which only field attributes are used without using any character attribute.
Although only three bits are included in each of the character attribute portions in the above prior technique, it would generally be necessary at present to allocate many more bits as a character attribute for determining various display modes, such as a reverse display, a high intensity display, a blinking display, a display with underlining, etc., which would aggravate the waste of memory space.
Further, the above prior technique using the special code format would not be well suited for use in ordinary information processing in eight-bit bytes and would require a special bus for transmitting the information.